A battery charger is a device used to apply power, a charge, to a rechargeable battery. The charge is stored in the battery until it is then drawn on by a complementary power-consuming unit to which the battery is attached. Battery chargers are available for simultaneously charging multiple batteries. Some chargers are capable of simultaneously cycling through different charging sequences to simultaneously charge different types of batteries.
One such battery charger is disclosed in the Applicant's U.S. patent application Ser. No. 09/102,142, filed Jun. 22, 1998, now U.S. Pat. No. 6,018,227, entitled BATTERY CHARGER ESPECIALLY USEFUL WITH STERILIZABLE, RECHARGEABLE BATTERY PACKS, which is incorporated herein by reference. In the system disclosed in this document, each battery or complementary charging module is provided with a memory. This memory stores data that indicates: the currents that should be applied to the battery; the test parameters that indicate whether or not the battery is charged; and the sequence in which the different currents should be applied to the battery and tests performed on the battery to evaluate its charge state. These data are read by a complementary processor internal to the battery charger. Based on these data, the battery charger processor directs the other components internal to the charger to: apply current to the battery and perform specific charge state tests on the battery in a specific sequence; apply the current to the battery at a specific level; and provide an indication of the charge state of the battery. This system is thus able to simultaneously apply charging currents to different batteries in sequences that are appropriate for each battery. This system eliminates the need to provide a charger for each new battery that has a charging sequence or other charging characteristics different from those of the other batteries at the facility at which the batteries are used.
The above-described system is well suited to simultaneously energize a number of different batteries. Nevertheless, there are some limitations to the utility of this system. One limitation is associated with the amount of power that the charger is able to provide to a number of batteries at any given instant. Clearly, the charger's ability to provide power to the batteries is limited by the amount of power outputted by the power supply internal to the charger. For example, one charger is provided with an internal power supply capable of providing 120 Watts of power to the complementary batteries. This particular charger can simultaneously charge four batteries. More particularly, the batteries this charger is intended to charge, at any given instant, will not draw more than 30 Watts. By limiting the number of batteries this charger can charge, it is inherently designed to not be placed in a state in which it is required to provide more power than it can deliver.
However, for reasons of efficiency, other chargers are designed to charge batteries that, collectively, may require more power than can be delivered by the charger power supply. For example it is desirable to provide a charger capable of charging six or more batteries. More particularly, it is anticipated that the normal maximum power required to charge each battery be in the range of 10 to 15 Watts. However, in order to maximize the utility of the charger, it should also be capable of charging batteries that draw as much as 30 Watts. To ensure that the charger be able to simultaneously provide sufficient power to charge all the batteries coupled to it, the charger would require an internal power supply capable of generating 180 Watts. However, the costs associated with providing a power supply capable of this output are noticeably greater than the costs associated with providing a power supply that provides a maximum of 120 Watts. Given the infrequency with which the charger is expected to deliver more than 120 Watts, providing a higher output power supply becomes economically inefficient. Moreover, power supplies capable of generating more than 120 Watts of power are typically appreciable larger in size than their low power alternatives. High output power supplies also generate more waste heat than low power supplies. For the above reasons, it is sometimes desirable to provide a battery charger with an internal power supply that is not able to supply all the power drawn by the batteries that could be connected to it.
Still another inefficiency associated with presently available charges is related to the consideration that, some batteries, after a given number of chargings, need to be completely recharged. This is because of voltage depression. "Voltage depression" is the inherent effect of a NiCd cell which causes a decrease in the cell's capacity to store charge due to the cell not being fully discharged.
In order to facilitate the discharge of batteries, the modules in which they are inserted have been provided with load resistors. Based on the sequence data associated with a battery, the battery is tied across the complementary load resistor. The load resistor serves as a sink for the charge stored in the battery. A disadvantage of this discharge system is that owing to the physical constraints of the size of the module size, the load resistor in the module is also small sized. Due to the small size and power limitation of the load resistor, it can take a lengthy period of time for the battery to fully discharge. Moreover, there may be a situation in which more than one battery needs to be discharged at the same time. In this situation, multiple batteries will discharge their power through the associated module-mounted load resistors. Cumulatively, these resistors will then generate significant amounts of heat. This heat can significantly warm the outer surfaces of the charger and the modules. At a minimum, this heat is simply radiated so as to turn the charger into source of unwanted waste heat. In some circumstances, this heat could warm the charger to a temperature at which a person touching it, or the batteries coupled to the charger, could suffer a burn injury.